1. Field of the Invention
The present invention is related to the field of voice data transmission through a network, and more specifically to devices, software, and methods for measuring a burstiness of packet loss in determining a quality of voice for network telephony.
2. Description of the Related Art
Voice transmissions, such as telephone calls, are increasingly made over packet switched networks. Unlike traditional telephone lines, networks break down the voice into data, arrange the data in distinct packets, and send the packets across the network. The voice is reconstructed at the other end of the voice call for the listener.
Sometimes packets can be lost. This can happen when packets are dropped from the network, or are delivered too late to be incorporated in the reconstructed voice sequence. In that case, the packets are absent, but are generally called lost packets in the art.
When a packet is lost, that means that a portion of the voice message does not reach the listener. In its place, the listener hears a small interruption, or the voice is syncopated. Thus, losing packets degrades the quality of the voice call, and thus of the telephone service.
There have been efforts to rectify at least the problem of low call quality. For example, when a single packet is lost, its data can be reconstructed from its immediately previous data. So, even though its information may be actually lost, the user will not hear an interruption. This is called packet loss concealment.
Packet loss concealment is harder to perform, however, when many packets are lost in a large group. They are harder to reconstruct, because for many the immediately previous data is not available.
In addition, there have been efforts to monitor the quality of service, so that other corrective measures can be taken. These efforts include quantifying the quality of service, in other words, determining how poor the service is at any given time. Once service is determined to be poor, other corrective measures can be taken.
Quality of service is quantified by measuring other parameters in addition to packet loss. These other parameters are delay, echo, codec degradation, etc. All the parameters are taken together to determine a total voice quality statistic, or figure of merit.
Packet loss is traditionally measured in the prior art as a rate R according to Equation (1):R=(number of lost packets)/(total number of packets)  Equation (1)
Equation (1) is used to determine voice quality. By using Equation (1), however, the prior art makes a fundamental assumption about the nature of packet loss. The assumption is that packet loss is distributed uniformly over the duration telephone call.
The assumption of the prior art does not take into account the true nature of packet loss. Packet loss is not distributed uniformly over the duration telephone call. Instead, it tends to come in bursts, or groups. Worse, some bursts last longer than others. Yet the present systems do not detect that, which is a deficiency.
The deficiency of the prior art is illustrated with reference to FIG. 1. Three waveforms A, B, C, are given for three sample voice data transmissions, all of the same time duration. The waveforms A, B, C illustrate packets PR as they are received, prior to any data reconstruction for packet loss concealment.
Each of the three waveforms A, B, C, reflects a sequence as reconstructed. Each sequence leaves blanks PL for absent packets PL. Packets are absent either because they are lost, or simply arrive too late to be incorporated in the play out.
In all three waveforms A, B, C, one fifth, or 20% of the packets are not received. These are indicated as lost packets PL. So, for all three waveforms A, B, C, Equation (1) returns R=20%. This yields the same voice call quality grade for all three. In the example of FIG. 1, that grade is “FAIR”.
It will be appreciated that the lost packets PL are grouped differently in each of waveforms A, B, C. In waveform A, the lost packets PL are distributed uniformly over the examined duration of the voice call. More particularly, every fifth packet PL is missing. In waveform B, the lost packets PL occur in somewhat bigger bursts. As drawn, every ninth and tenth packet PL are missing. In waveform C, all 10 packets are lost in one big burst.
In reality, for waveform B, the perception of the voice call quality will be somewhat worse than in waveform A, while the perception for waveform C will be by far the worst. That is because it will be harder to reconstruct the missing packets, since they occur in bursts. As such, waveform B deserves a worse grade than waveform A. For the same reason, waveform C deserves an even worse grade. That is because packet loss concealment is the hardest to perform for waveform C. Many of the immediately previous data is not available, and the voice will be perceived as lost.
The problem, however, is that the differentiation in eventual quality of the reconstruction is not predicted by the grading system.